I don't know what Yamaha receiver you have, but I seem to recall from reading my manual (mine is a RX-V750, which is a 7.1 receiver), that they have a specific recommendation for setting it up as a 6.1 system, with a single "back-surround" speaker. I don't think it is done by wiring to both back-surround binding posts, however. My recollection is that you just use one (probably the left back-surround) set of binding posts, and then when you set up the receiver (or run auto-set-up, if it has that feature) you identify the other back-surround as "off", and then the receiver knows that there is only one back-surround, and operates in 6.1 mode.
If for some reason you don't have your manual, you can always download a .pdf file from yamaha's web site.